Tensers is such a short duration spell that I dont see getting a lot of mileage out of being able to cancel it. It may be convenient but not a game breaker. As for scrolls somehow being overpowered with TT, if that were the case then people would look for UMD rogues rather than casters when they need magic.
We really are left with 2 options concerning Tensers:
improve the spell performance and let it continue preventing spell casting
OR
allow casting and other magic use while the spell is active.
I personally feel a performance increase is closest to the spirit of the spell but casting while it active is a solid second option.